1. Field of the Invention
The present invention relates to heading sensors, and in particular to heading sensors for use in automatic vehicle location systems.
2. Description of the Prior Art
Vehicle location monitors often utilize heading sensors to determine the heading of a vehicle with respect to a reference direction. Heading sensors generally utilize a magnetic compass or inertial sensors. However, magnetic compass sensors are subject to errors caused by magnetic field anomalies resulting from buildings, bridges, and other vehicles. Inertial type heading sensors are typically too expensive for large scale use in automotive vehicle location systems. In addition, various systems utilize "resettable relative heading sensors" wherein heading is determined with respect to a reference direction which is periodically changed. One such system is described in the aforementioned copending application Ser. No. 694,042. Compass and inertial type heading sensors are disadvantageous for use as resettable relative heading sensors because relative heading sensors utilizing compass or inertial type sensors require additional digital logic circuitry to determine and store the reference direction.
Heading sensors, utilizing essentially digital components, wherein the heading with respect to the reference direction is determined from the difference in distance traveled by laterally aligned wheels (hereinafter "lateral wheels"), for example, the right and left front wheels, are well known in the art. Such systems are hereinafter referred to as "two odometer systems." Briefly, two odometer systems operate on the principle that during a turn by the vehicle the lateral wheels of the vehicle follow circular paths about a common center of curvature, with the respective radii of the paths differing by the distance (W) between the wheels. The difference in distance (.DELTA.D) traveled by the respective wheels is expressed mathematically as follows: ##EQU1## where .DELTA..theta. is the angular amount of turn in degrees. Accordingly, the angular amount of turn is mathematically expressed as: ##EQU2##
A description of one such two odometer system is found in "Urban Vehicle Monitoring: Technology, Economics, & Public Policy" Vol. II: Technical Analysis & Appendices, Final Report. prepared by Teknekron Inc., for the U.S. Department of Housing and Urban Development under Contract H-1030, Oct. 31, 1970. The Teknekron two odometer system utilizes the total accumulated difference in distance traveled by the respective wheels to represent the instantaneous heading of the vehicle. Thus, the distance difference is continuously integrated to produce a signal indicative of the instantaneous heading. Such a system is disadvantageous in that small heading errors due to, for example, differences in wheel circumferences, and lane changes, are cumulative.
Another two odometer system is described in U.S. Pat. No. 3,845,289 issued Oct. 29, 1974 to French et al. The French et al. system, however, operates only to indicate that a turn through an angle beyond a predetermined threshold value has been made by the vehicle and the direction (right or left) of such turn. No provision is made for determining the angle turned through by the vehicle.